1. Field of Invention
This invention relates to an image recording apparatus, and more particularly to a contiguous microlens array, a method of fabricating the same and a photomask for defining the same, which can be applied to any image recording apparatus that requires focusing light on photosensing devices.
2. Description of Related Art
In a semiconductor-type image recording apparatus like a charge-coupled device (CCD) or CMOS image recording apparatus, a microlens array is often disposed over the array of photosensing devices to enhance the sensitivity of the same, wherein one microlens focuses light on one photosensing device.
FIG. 1A is a local contour plot of a conventional microlens array, and FIG. 1B shows the height variations of one microlens in two vertical cross-sectional views at different angles. Because a microlens 110 is formed by reflowing a square photoresist pattern formed on a based layer 10, the shapes of the lower contours thereof are close to squares, as shown in FIG. 1A, so that the microlens 110 has different curvatures in vertical cross-sectional views at different angles. For example, the curvature of the 45° cross section (B-B′) is relatively smaller than that of the 0° cross section (A-A′), as shown in FIG. 1B, so that the microlens 110 is insufficient in the focusing effect. Moreover, because neighboring microlenses 110 are not connected with each other and there are planar sections without a focusing effect between them, the incident light are not fully collected so that the light focusing is not quite effective.
There is also an issue on the integration of the conventional microlens array with other elements in a image recording apparatus, which is described below with a CMOS image recording apparatus having photodiodes as photosensing devices as an example. Referring to FIG. 2 schematically showing a part of a CMOS image recording apparatus in the prior art, the microlens array 100 is formed on a transparent base layer 10, which includes a color filter array 12 and other functional layers on a multi-level interconnect structure 20 including a first-level interconnect layer 22 and a second-level interconnect layer 24 over a photodiode array 30. The eyepiece 40 of the CMOS image recording apparatus is disposed above the microlens array 100, apart from it by a certain distance.
Because the incident angle of the light incident to a microlens 110 in a peripheral portion of the microlens array 100 overly deviates from 90° (the direction of 90° means the normal line direction of the image sensor chip, hereinafter) so that the focus of light is not directly under the microlens 110, the microlens 110 is laterally shifted relative to the corresponding photodiode 30 to make the light focus on the latter, as shown in FIG. 2. However, this makes the exit light 50a from the microlens 110 partially blocked by the second-level interconnect layer 24 and thus lowers the recording accuracy of the image. This problem can be solved by laterally shifting portions of the 2nd-level interconnect layer 24 under the peripheral part of the microlens array 100, but the interconnect circuit design would become more complicated by doing so.